This invention relates to the feeding of ribbons carrying ink for printing and in particular to the feeding of thermal transfer printing ribbons.
Ribbons are utilised to carry a supply of ink to a printing station at which printing elements cause ink to be transferred from the ribbon to the surface of an article upon which printing is to be effected. In thermal transfer printing processes, the ink is carried in a layer on the surface of a ribbon substrate and the ink is transferred from the ribbon to the surface of the article by the application of heat to the ribbon. With commonly available thermal transfer ribbons, a total transfer of ink occurs at each printing operation so that the ribbon can be used only once. If printing is effected in a serial manner in which the article is moved past the printing elements and the required print impression is built up in a series of printing operations, the ribbon is fed with and at the same speed as the feeding of the article past the printing station. On the other hand if the required impression is printed in a single printing operation in which the article is held stationery at a printing station, the ribbon is also held stationery during the printing operation and is fed during the interval between consecutive printing operations. In each case, the ribbon is fed by approximately the length of print impression for each printing operation. Because the ink corresponding to the pattern of the print impression is totally removed from the ribbon during a print operation the ribbon cannot be re-used because areas of the ribbon from which ink has been totally removed may coincide with areas of the ribbon required for printing a subsequent impression. If such a ribbon was re-used the printing would be defective and unreliable. The ribbon is usually contained on a reel or spool from which it is unwound in use and rewound onto another reel or spool. When the total length of the ribbon has been used, it is necessary to replace the ribbon with a new unused ribbon and this results in an interruption in the use of the printing device. In order to allow a ribbon to be used for a larger number of printing operations multi-strike ribbons are being developed. In one form of these multi-strike ribbons the ink is contained in a plurality of layers. Only a portion of the ink, for example one layer, is removed in the course of a printing operation whereby the length of ribbon may be used a number of times. However while this provides a saving in terms of replacing the ribbon, after each transit of the length of the ribbon past the printing station the ribbon needs to be rewound prior to the next transit of the ribbon. It will be appreciated that because thermal transfer ribbons need to be fed in the same direction as the article on which printing is to be effected, it is not practicable to reverse the direction of feed of the ribbon for each successive transit of the ribbon. Consequently, when the end of the ribbon is reached, subsequent printing operations are delayed by the time taken to rewind the ribbon. A further difficulty arises from the flimsy nature of the ribbon substrate. The substrate tends to become distorted during printing operations and it is difficult to achieve satisfactory rewinding of the ribbon without damage occurring to the substrate of the ribbon.